This invention relates to hydrogen ion, or pH, sensors. Sensors are employed to determine the content of a specific substance in a fluid or atmosphere. For example, a sensor can be employed to determine the content of oxygen, or carbon dioxide in a sample, or its content of hydrogen ions or other ions in solution.
Ion sensors are known in the prior art for measuring the hydrogen ion activity or pH of a sample. Such a sensor, for example the well-known glass electrode, is used in conjunction with a reference electrode. When both are immersed in a solution the potential difference between the two electrodes is a function of the concentration of hydrogen ions in the solution.
Sensors such as those previously described are normally utilized for measuring ion concentrations of liquids at approximately ambient temperature conditions, or perhaps at somewhat higher temperatures ranging up to about 100.degree.-150.degree. C. levels. However, in connection with certain recent innovations, such as nuclear reactors, geothermal wells and other operations entailing relatively high temperature fluid systems, there has often been interest in, or a need for, measuring the pH of water at temperatures in the order of about 300.degree. C., or higher.
Conventional glass-containing electrodes are not suitable for such high temperature service because of the accelerated rates of attack and dissolution of glass occurring at such temperatures. Other pH sensors based upon reversible electrode couples, for instance the conventional hydrogen electrode, the oxygen electrode, the palladium hydride electrode and various metal--metal oxide couples, although potentially useful under certain limited conditions, lack the general versatility of electrodes having glass membranes. Accordingly, some sensor systems require the presence of reactive gases in known concentrations, many systems are poisoned by impurities such as sulfides and most sensor systems are influenced by other reduction--oxidation couples in the system.
One of the virtues of a membrane in a membrane type electrode (e.g. the glass electrode) is that, unlike the above constructions, they are normally insensitive to changes in the reduction-oxidation environment and are resistant to poisoning by impurities such as sulfides. Membrane materials can often be found that exert no significant influence on the environment.